Due to their high storage density, long data retention life, and relatively low cost, optical disks are becoming increasingly popular as a means to distribute information. Large format disks have been developed for storing full length motion pictures. The compact disk (CD) format was developed and marketed for the distribution of musical recordings and has replaced vinyl records. High-capacity, read-only data storage media, such as CD-ROM and Digital Versatile Disk (DVD), have become prevalent in the personal computer field, and the DVD format may soon replace videotape as the distribution medium of choice for video information.
Recently, relatively inexpensive optical disk writers and writable optical media have become available, making optical disks popular as backup and archival storage devices for personal computers. The large storage capacity of writable optical disks also makes them ideal for use in multimedia authoring and in other applications which require access to large amounts of storage. Current writable optical disk technologies include several write-once technologies, such as CD-Recordable (CD-R). A few technologies permit writing, erasing, and rewriting data on a disk, such as Mini-Disk (MD), which uses magneto-optical technology. Other writable formats employ phase-change and dye-polymer technology. Recent advances in writable optical disk technology have made rewritable optical media more practical, and the specification for DVD-RAM calls for use of high-capacity rewritable optical media. These writable and rewritable formats are expected to further increase the use of optical disks as a data storage solution for use with personal computers.
An optical disk is made of a transparent disk or substrate in which data, in the form of a serial bit-stream, is encoded as a series of pits in a reflective surface within the disk. The pits are arranged along a spiral or circular track. Data is read from the optical disk by focusing a low power laser beam onto a track on the disk and detecting the light reflected from the surface of the disk. By rotating the optical disk, the light reflected from the surface of the disk is modulated by the pattern of the pits rotating into and out of the laser's field of illumination. Optical and imaging systems detect the modulated, reflected, laser light and produce an electrical signal which may be decoded to recover the digital data stored on the optical disk.
To be able to retrieve data from an optical disk, the optical systems include an optical pickup assembly which may be positioned to read data on any disk track. Processor-driven servo mechanisms are provided for focusing the optical system and for keeping the optical pickup assembly positioned over the track, despite disk warpage or eccentricity.
Because in most previously known systems the data is read serially, i.e. one bit at a time, the maximum data transfer rate for an optical disk reader is typically determined by the rate at which the pits on the disk pass by the optical pickup assembly. The linear density of the bits and the track pitch (distance between tracks) are fixed by the specification of the particular optical disk format. For example, CD disks employ a track pitch of 1.5 .mu.m (.+-.0.1 .mu.m), while DVD employs a track pitch only about one-half as wide.
Previously known methods of increasing the data transfer rate of optical disk readers have focused on increasing the rate at which the pits pass by the optical pickup assembly by increasing the rotational speed of the disk itself. Currently, drives with rotational speeds of up to 16x standard speed are commercially available, and even faster reading speeds have been achieved by moving to constant angular velocity designs. Higher disk rotational speeds, however, place increased demands on the optical and mechanical subsystems within the optical disk drive, create greater vibration, and may make such drives more difficult and expensive to design and manufacture.
A cost effective alternative to increasing the disk rotational speed to provide faster optical disk readers is to read multiple data tracks simultaneously. Numerous methods for generating multiple beams to read several tracks simultaneously have been used. U.S. Pat. No. 5,144,616 to Yasukawa et al., for example, shows an array of laser diodes which may generate multiple beams for use in simultaneously reading multiple tracks of an optical disk. U.S. Pat. No. 4,459,690, to Corsover, uses acousto-optical techniques to split a beam into multiple beams for use in reading an optical disk. Other systems have used a diffraction grating to generate multiple beams used to simultaneously illuminate multiple tracks. The system described in commonly assigned U.S. Pat. No. 5,426,623, to Alon et al., uses a wide area illumination beam to simultaneously read multiple tracks of an optical disk.
Using a system which reads multiple tracks simultaneously may provide for dramatic increases in the speed of optical disk readers. For example, a drive which rotates the disk at only eight times the standard speed (i.e. an 8X drive), and reads seven tracks simultaneously, may provide reading speeds equivalent to a true 56X drive.
It should be noted that as used herein, a data track is a portion of the spiral data track of a typical optical disk, and follows the spiral for one rotation of the disk. Thus, a drive capable of reading multiple data tracks simultaneously will read multiple portions of the spiral data track at once. For optical disks having concentric circular tracks, a data track would refer to one such circular track. For disks having multiple concentric spiral tracks, such as those described in commonly assigned, copending U.S. patent application Ser. No. 08/885,425, filed Jun. 30, 1997, a data track would refer to one of the concentric spiral tracks.
Implementation of simultaneous multiple track reading capability for optical disks presents new design challenges. If multiple beams are used, for example, the beams must be properly aligned with the tracks being read, and the beams reflected from the optical disk must be correctly aligned with the photodetector. Manufacturing tolerances may lead to minor differences in magnification of an optical pickup assembly, leading to minor differences in the spacing of the beams between systems. These errors may result in a portion of the light beam reflected from one track being read by a photodetector associated with a neighboring track.
Additionally, there is some variation in the track pitch allowed in the specification of most optical disk formats, such as the CD and the DVD formats. A multi-beam optical disk reader using such a format must be able to detect and correct for these magnification errors and track pitch variations to insure that the beams are correctly aligned with the tracks. Such errors arising from track pitch variations also may result in a portion of the light beam reflected from one track being read by a photodetector associated with a neighboring track.
It would therefore be desirable to provide apparatus and methods that enable detection and correction of magnification errors arising due to optical system manufacturing variations, and that enable cross-talk components arising due to such magnification errors to be corrected.
It further would therefore be desirable to provide apparatus and methods that enable detection and correction of magnification errors arising due to track pitch variations, and that enable cross-talk components arising due to such magnification errors to be corrected.